1. Field of the invention
The present invention relates to a high thermal conductive silicon nitride sintered body and a method of producing the same. More particularly, the invention relates to a high thermal conductive silicon nitride sintered body which achieves high thermal conductivity and good heat-radiating characteristics, as well as the high strength characteristics generally inherent in silicon nitride, so as to be suitable for semiconductor substrate, various types of radiator plates, etc., and a method of producing the high thermal conductive silicon nitride sintered body.
2. Description of the Related Art
Ceramic sintered bodies containing silicon nitride as a main component have strong heat resistance. They resist temperatures as high as 1000.degree. C. or higher. Silicon nitride ceramic sintered bodies also have strong thermal shock resistance due to their low thermal expansivity. Because of these characteristics, silicon nitride ceramic sintered bodies are expected to be widely used as high-temperature structural materials, most of which are currently made of heat-resistant super alloys. In fact, silicon nitride ceramic sintered bodies are already used for high-strength heat-resistant components and parts of, for example, gas turbines, engines or steel making machines. Further, because of their high corrosion resistance to metal, some silicon nitride ceramic sintered bodies are applied to melt-resistant material for molten metal. Still further, because of their high abrasion resistance, some silicon nitride ceramic sintered bodies are applied to or tested for cutting tools or sliding parts such as bearings.
Various sintering compositions for silicon nitride ceramic sintered bodies are known, for example: silicon nitride-yttrium oxide-aluminum oxide system; silicon nitride-yttrium oxide-aluminum oxide-aluminium nitride system; and silicon nitride-yttrium oxide-aluminum oxide-oxide of titanium, magnesium or zirconium.
The oxides of rare earth elements, such as yttrium oxide (Y.sub.2 O.sub.3) in the sintering compositions listed above, have been widely used as sintering assistant agents. Such rare earth element oxides enhance the sintering characteristics of sintering materials and, therefore, achieve high density and high strength of the sintered products (sintered bodies).
According to the conventional art, silicon nitride sintered bodies are generally mass-produced as follows. After a sintering assistant agent as mentioned above is added to the powder of silicon nitride, the mixture is molded to form a compact. Then, the compact is sintered in a sintering furnace at about 1600.degree.-1850.degree. C. for a predetermined period of time followed by cooling in the furnace.
However, though the silicon nitride sintered body produced by the conventional method achieves high mechanical strengths such as toughness, the thermal conductivities thereof are significantly lower than those of aluminum nitride (AlN) sintered bodies, beryllium oxide (BeO) sintered bodies or silicon carbide (SIC) sintered bodies. Therefore, conventional silicon nitride sintered bodies are unsuitable for electronic materials, such as semiconductor substrates, that need good heat-radiating characteristics. Accordingly, the use of silicon nitride sintered body is thus limited.
Aluminum nitride sintered bodies have high thermal conductivity and low thermal expansivity, compared with other ceramic sintered bodies. Aluminum nitride sintered bodies are widely used as packaging materials or materials of circuit base boards for semiconductor chips, which have been progressively improved in operational speed, output power, variety of functions and size. However, no conventional aluminum nitride sintered bodies achieve sufficiently high mechanical strengths.
Therefore, there is a growing need for a ceramic sintered body having both high thermal conductivity and high strength.